Butter a baking dish. Cut bread into ½-inch slices and arrange in baking dish, overlapping the edges of the slices.
In a large bowl, whisk together eggs, milk, vanilla, sugar, lemon zest, cinnamon and salt.
Pour mixture over the bread and allow to sit for 30 minutes, or up to 24 hours, covered with plastic wrap in the refrigerator.
Preheat oven to 325°.
Sprinkle blueberries around and between the egg mixture–soaked bread. (Do this just before baking, because doing it ahead of time will stain the bread purple and turn the egg mixture green.) Bake for 40–45 minutes, or until golden brown and the custard is set. If it begins to brown too quickly, cover with foil.
Let cool at least 15 minutes before serving. Can be served at room temperature, cold or warm. Top with your favorite syrup, butter and powdered sugar.